Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 859
claimed but has never been proven. For example, Hayek et al.
were able to synthesize the oxide sulfates Nb2O(SO4)4, MoO-
(SO4)2, and WO(SO4)2 and the sulfate Ta2(SO4)5 as white,
extremely hygroscopic powders by reaction of the corre-
sponding metal chlorides with SO3 in sulfuryl chloride.6,7
Ta2(SO4)5 was also obtained by Goroshchenko and Andreeva
in the system Ta2O5/H2SO4/SO3.8 The element distribution in
these compounds was determined using elemental analysis or
thermogravimetric measurements. There are no reliable struc-
tural data available.
25% SO3 (puriss. Merck, Darmstadt, Germany) was heated to
200 °C for 48 h and then cooled to room temperature at a cooling
rate of 2.5 °C/h. In this way, colorless single crystals of Nb2O2-
(SO4)3 suitable for X-ray diffraction could be grown. The glass
ampule was opened in a glovebox under a nitrogen atmosphere,
and the mother liquor was separated from the crystals by
decantation. The crystals for X-ray diffraction were selected
under inert oil (perfluorinated polyether, ABCR, Karlsruhe,
Germany). For the DTA/TG measurements, the precipitate was
washed with absolute tetrahydrofuran and dried in vacuo to
remove the last traces of adhesive sulfuric acid.
Due to their high dielectricity constants, Nb2O5 and Ta2O5
are especially auspicious candidates for capacitor dielectric
materials in complementary metal-oxide semiconductors
(CMOS). New gate oxide materials are an essential step in
the realization of Moore’s Law of miniaturization in future
semiconductors. For this reason, thermally labile compounds
of the refractory elements have recently attracted intense
interest as precursors for the deposition of thin metal-oxide
layers. Actually, layers of Nb2O5 and Ta2O5 are prepared
using techniques like metal-organic chemical vapor deposi-
tion (MOCVD), atomic layer deposition (ALD), and pulsed
laser deposition (PLD).9 Typical compounds used in these
processes are volatile materials like chlorides, alkoxides, and
more recently amides.10,11 The big disadvantage of these
methods is contamination of the deposited oxide layer with
remarkable amounts of impurities, especially carbon.
In the search for carbon-free precursors for refractory
metal oxides, we have started to extend our knowledge of
the chemistry of precious metals and rare earth elements with
complex oxo-anions.12-15 Hence, we began investigating
compounds of the refractory elements containing complex
oxo-anions such as nitrates, perchlorates, and sulfates. Here,
we report the crystal structure and the thermal decomposi-
tion of new oxide sulfates of refractory elements with the
examples of Nb2O2(SO4)3, WO(SO4)2, and two modifica-
tions of Re2O5(SO4)2. Furthermore, we provide a new
synthetic approach to MoO2(SO4) leading to well-developed
single crystals suitable for X-ray crystallography.
MoO2(SO4). A mixture of 1 g of MoO3 (99.9%, Alfa-Aesar,
Karlsruhe, Germany) and 1.5 mL of concentrated sulfuric acid
€
(pure, BuFa, Oldenburg, Germany) was heated to 300 °C for
72 h followed by slow cooling to room temperature (2.5 °C/h).
For satisfying crystal growth, it is essential that the water
generated during the reaction is able to pass out of the system,
which is easily achieved by leaving the ampule open to the
atmosphere. In this way, colorless needles of several millimeters
in length can be grown. Crystals for X-ray diffraction and the
material used in DTA/TG measurements were prepared as
described above for Nb2O2(SO4)3.
WO(SO4)2. A solution of 0.5 g of WOCl4 in 1 mL of fuming
sulfuric acid containing 65% SO3 (puriss. Merck, Darmstadt,
Germany) was heated to 200 °C for 48 h. Slow cooling of the
resulting mixture to room temperature (2.5 °C/h) leads to small
colorless single crystals. Crystals for X-ray diffraction and the
material used in DTA/TG measurements were prepared as
described above for Nb2O2(SO4)3. The reactant WOCl4 was
prepared according to literature methods by heating a suspension
of WO3 (99.8%, Alfa-Aesar, Karlsruhe, Germany) in thionyl
chloride to 200 °C in a sealed glass ampule for 8 h.16 After
evaporation to dryness, the WOCl4 was purified through sub-
limation under reduced pressure.
Re2O5(SO4)2-I. A suspension of metallic Re (H. C. Starck,
Goslar, Germany) in fuming sulfuric acid (65% SO3) was heated
to 200 °C. On cooling, a small amount of pale yellow crystals
separated from the mother liquor. A more convenient synthesis
is the reaction of 0.75 g of Re2O7(H2O)2 (“solid perrhenic acid”)
with 2 mL of fuming sulfuric acid (65% SO3) at 200 °C followed
by slow cooling to room temperature (2.5 °C/h).
Re2O5(SO4)2-II. A solution of 0.5 g of Re2O7(H2O)2 in 1 mL
of fuming sulfuric acid (65% SO3) was heated to 300 °C followed
by slow cooling to room temperature (2.5 °C/h). In this way,
yellow crystals of several millimeters in length can be grown. For
both modifications, crystals for X-ray diffraction and the
material used in DTA/TG measurements were prepared as
described above for Nb2O2(SO4)3. Re2O7(H2O)2 was prepared
according to literature methods by dissolving Re metal in 30%
H2O2 and evaporating it to dryness.17
2. Experimental Section
2.1. Synthesis. All reactions were carried out in sealed glass
ampules of 25 cm in length and 2 cm in diameter, which were
heated in a block thermostat (Gefran 800P, Liebisch, Bielefeld,
Germany), allowing the programming of designated heating
and cooling rates. The glass ampules were charged with the
starting materials in a glovebox under an inert atmosphere and
then sealed under vacuum conditions with a CH4/O2 flame.
Nb2O2(SO4)3. A solution of 0.5 g of NbCl5 (99%, Alfa-Aesar,
Karlsruhe, Germany) in 2 mL of fuming sulfuric acid containing
2.2. X-Ray Crystallography. The selection of suitable single
crystals of all four compounds was performed under inert oil.
Afterward, the crystals were mounted inside an oil drop on a
glass capillary (0.1 mm in diameter) and were placed into a
stream of cold nitrogen (-120 °C) inside the diffractometer
(IPDS I, Stoe, Darmstadt, Germany or κ-APEX II, Bruker,
Karlsruhe, Germany). After checking the crystal quality and
determining the unit cell parameters, the reflection intensities
were collected. Table 1 presents crystallographic information
for Nb2O2(SO4)3, MoO2(SO4), WO(SO4)2, Re2O5(SO4)2-I,
and Re2O5(SO4)2-II. Further details of the crystal structure
investigations may be obtained from Fachinformationszentrum
Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax:
(þ49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de;
(6) Hayek, E.; Hinterauer, K. Monatsh. Chem. 1951, 82/2, 205–209.
(7) Hayek, E.; Engelbrecht, A. Monatsh. Chem. 1949, 80/5, 640–647.
(8) Goroshchenko, Y. G.; Andreeva, M. I. Russ. J. Inorg. Chem. 1965, 10/
4, 517–519.
(9) Chaneliere, C.; Autran, J. L.; Devine, R. A. B.; Balland, B. Mater. Sci.
Eng. 1998, 22, 269–322.
(10) Maiti, C. K.; Samanta, S. K.; Chatterjee, S.; Dalapati, G. K.; Bera,
L. K. Solid-State Electron. 2004, 48, 1369–1389.
(11) Hellwig, M.; Milanov, A.; Barreca, D.; Deborde, J.-L.; Thomas, R.;
Winter, M.; Kunze, U.; Fischer, R. A.; Devi, A. Chem. Mater. 2007, 19,
6077–6087.
(12) Wickleder, M. S. Z. Anorg. Allg. Chem. 2001, 627, 2112–2114.
(13) Pley, M.; Wickleder, M. S. Z. Anorg. Allg. Chem. 2004, 630, 1036–
1039.
(16) Hecht, H.; Jander, G.; Schlapmann, H. Z. Anorg. Chem. 1947, 254,
255–264.
€
(14) Wickleder, M. S.; Buchner, O.; Gerlach, F.; Necke, M.; Al-Shamery,
€
(17) Brauer, G. Handbuch der praparativen anorganischen Chemie Bd. III;
K.; Wich, T.; Luttermann, T. Chem. Mater. 2008, 20, 5181–5185.
(15) Wickleder, M. S. Z. Anorg. Allg. Chem. 2000, 626, 1468–1472.
Ferdinand Enke Verlag: Stuttgart, Germany, 1981.
Betke, Ulf
